rubble trench foundation for grill

I am working on a design for a masonry grill that I would like to build this coming summer. It will probably be similar to the one published by TMEN. The total size of the footprint will be about 3' x 3'. Everything that I have found online regarding rubble trench foundations is for a much larger structure. Is there anything that I need to take into consideration when designing a foundation for something this small?

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